Background and purpose: Renal cell carcinoma (RCC) is well known for its chemoresistance. The membranous p-glycoprotein (gp-170) is believed to be highly correlated with multidrug resistance (MDR) of cancer cells with energy-dependent pumping efflux of anticancer drugs. Verapamil, a calcium antagonist, inhibits the efflux function of gp-170 and cytoskeletal transportation. The aim of this study was to determine the effect of verapamil on gp-170 expression and intracellular drug accumulation in RCC tumor cells and the modulation of cytotoxicity of various chemotherapeutic drugs on native RCC cell lines and acquired MDR sublines by verapamil.
Methods: Using cultured cell lines of RCC and their MDR sublines as target cells, the effect of verapamil on gp-170 expression was analyzed by immunofluorescence flow cytometry. The influence of verapamil on intracellular drug accumulation in RCC tumor cells was measured by autofluorescence flow cytometry. The modulation of verapamil on cytotoxicity of various chemotherapeutic drugs on native RCC cell lines and acquired MDR sublines was analyzed by the methyl tetrazolium method.
Results: From flow cytometric measurement, the expression of gp-170 was significantly decreased in A704 and Caki-1 tumor cells after verapamil treatment. The uptake of adriamycin and maintenance of intracellular drugs were also significantly increased following verapamil treatment in RCC8701 tumor cells. These effects were sustained for as long as 8 hours after verapamil withdrawal. The cytotoxicity of adriamycin and epirubicin on RCC8701 and its MDR subline tumor cells was markedly intensified by verapamil. The verapamil modulation of cytotoxicity was in an immediate-reaction pattern and was dose-dependent, with synergistic effects. Long-term treatment was more effective than short-term treatment in RCC MDR sublines. The cytotoxicity of vinca alkaloid (vinblastine) and alkylators (carboplatin) was also enhanced by verapamil.
Conclusions: These results suggest that verapamil plays an important role in the circumvention of native and acquired chemoresistance of RRC because it suppresses membranous gp-170 expression and cytoplasmic drug transportation.